It has been almost a century since the United States walked away from an international accord as significant as the Paris agreement on climate. In 1920 the Senate refused to ratify the Treaty of Versailles despite the desperate attempts by President Woodrow Wilson to rally support for it.

The Versailles treaty brought an end to the First World War, and historians agree that in many ways it wasn't a very good treaty. The British and the French were more interested in punishing Germany than in setting terms for a post-war peace, and Germany plunged into economic and political crisis in part as a result of the treaty.

But Wilson's primary objective when he went to France was to create the League of Nations, an international organization where future disputes could be negotiated before they exploded into another cataclysmic war. Republican leaders in the Senate did not want the United States to join the League, and that is the primary reason they killed the treaty.

We all know what happened next. Within a dozen years Germany had been taken over by the Nazi party and Europe was on its way to war again. Wilson saw it coming. As he lobbied for the League of Nations in 1919, he predicted "with absolute certainty that within another generation there will be another world war if the nations of the world do not concert the method by which to prevent it."

Walking away from the Treaty of Versailles and the League of Nations created one of those great historical "what if's." Had the United States joined the League, could World War II have been prevented? Would events in Europe have gone differently had this country not retreated from the world stage into its splendid isolationism? Impossible to say.

But we do know that without the United States the League of Nations proved to be less effective than it might have been. We also know that the isolationist reflex that led to the rejection of the League also prompted Congress to pass a series of extremely restrictive — xenophobic, really — limits on immigration. Those restrictions, in turn, were used to bar Jewish refugees from entering the country during the 1930s.

The price of isolationism was this: As Europe descended into madness, the United States stuck its head deeper into the sand. We pretended it wasn't happening; we convinced ourselves it wasn't our problem. But it was happening, and it turned out to be our problem after all.

What comes next now that Donald Trump has pulled us out of the Paris Accords is anyone's guess. Perhaps the Chinese and the Germans will now assume global leadership in developing renewable energy and international development. After being insulted by Trump, the Germans seem only too eager to work with the Chinese. Perhaps American consumers and states like California and New York will force progress on carbon reduction despite the feckless, head-in-the-sand "leadership" we have right now in Washington. Maybe other nations will follow our lead and pull out of the climate agreement as well, and the planet will heat up even faster.

But we know for sure that the climate is warming up, and that climate change is already causing instability around the globe. And we also know that by walking away from the Paris goals, the United States just diminished its position of international leadership. In his second inaugural address Bill Clinton called the United States "the indispensable nation." Trump apparently doesn't see the United States this way.

We can't know what might have happened had the United States ratified the Treaty of Versailles, but one lesson seems clear. Isolationism is simply another name for ignoring inconvenient truths.

As we leave the Paris Accords, we move as a nation from indispensable to increasingly irrelevant, and the neither the world nor this country will be better off as a result.

Steven Conn is the W. E. Smith professor of history at Miami University in Oxford, Ohio.